I. Field of Invention
This invention relates generally to the control of toggle type injection molding machinery. In particular, the invention provides an improved velocity control of a toggle clamp mechanism during toggle lockover.
II. Description of the Prior Art
Typically, the first step in an injection molding machine cycle of operation is to close and lock the clamp, i.e., to bring mold elements mounted on movable and stationary platens in a closed relationship under a predetermined clamping force. Generally, a toggle mechanism is connected to the movable platen and provides a mechanical advantage to a clamp actuator.
It is known to operate the clamp actuator and toggle mechanism and close the clamp according to a cycle of operation which is illustrated in FIG. 4a. FIG. 4a illustrates relative magnitudes of toggle mechanism velocity as a function of toggle mechanism position during a clamp close cycle. First, closure of the toggle clamp is initiated from an open position 200 at a predetermined high velocity as shown at 202. When the toggle mechanism reaches a predetermined position 204, the velocity is reduced to a lesser mold protect velocity 206 as shown at position 205. Toggle mechanism operation continues at the mold protect velocity to bring the platens and mold elements together. When the toggle mechanism reaches a mold touch position 208 at which point the mold elements are in a contacting relationship, the operation of the toggle mechanism is continued at a low velocity 210 until it achieves a locked position 212 at which point the toggle mechanism has an optimal mechanical configuration to resist mold separating forces created by the injection molding process.
In a continuing effort to improve the efficiency of the injection molding machine cycle of operation, the present invention proposes to increase the speed of operation of the clamp mechanism. Applicants have discovered one area for improvement with regard to the close clamp cycle, that being the operation of the clamp mechanism during the lockover portion of the cycle. Lockover is that portion of the clamp close cycle which is illustrated between positions 208 and 212 of FIG. 4a. After the molds have been closed, during lockover, the toggle is brought to its locked position which builds a predetermined clamping force to hold the molds together.
While it is desirable to maintain the highest possible lockover velocity, it is equally important that the toggle mechanism be brought to a stop without overshooting the lockover position. If the toggle mechanism actuator is a piston and cylinder, overshooting the lockover position will cause the piston to bottom on the cylinder not only potentially damaging the piston and cylinder mechanism, but also creating a disruptive and irritating banging noise. Depending on the toggle design, overshoot of the lockover position may also damage the toggle mechanism itself. In order to minimize the potential for damage to the toggle mechanism and its actuator, the toggle mechanism velocity during the lockover portion of the close clamp cycle is typically a relatively small velocity magnitude e.g. in a range of 10 to 20 percent of the maximum clamp velocity